Omega Centauri Brightness

[ixian (Luke)]

I just got myself a new 8 inch DOB and I was wondering how to improve the contrast when visually viewing Omega Centauri? I am viewing from a regional city and I can still make out the faint glow of the milky way but the sky is light polluted. Am I better just waiting for a dark site visit or can it be improved from the back yard? I feel like I really have to strain my eyes to stop the stars looking too dim. The object itself looks almost lattice like.

[astro744]

Increasing magnification will darken the background. If you have your usual 25 & 10mm eyepieces you will soon crave for something else. Which eyepiece are you using?

[ixian (Luke)]

The scope came with an illuminated 12.5 mm plossl and a 26mm 2 inch deepview. I also have a set of Meade 4000 super plossls. I don't know anything about eyepieces, I've been trying to figure it out but it makes me feel stupid. I understand focal lengths and magnifications but that is about all. The focal length of the scope is 1200mm and a 9.7 mm eyepiece appears to fill edge to edge on Omega.

[julianh72 (Julian)]

There's nothing like a dark site, but I find that a cheap Sky Glow / Light Pollution Filter really does make a noticeable improvement on the fainter objects (in suburban Brisbane).

The one I'm using partially attenuates the mercury and sodium wavelengths, with very little attenuation of other visible light wavelengths, so it really does cut back the sky-glow without dimming the other features significantly. You can see the effect very clearly if you just hold it up to the night sky, or look at a sodium or mercury street light at night, and I tested it with my Desktop Spectroscopy kit http://publiclab.org/wiki/spectrometer to see if it really does what it says on the box - and it does!

[Monstar (Johnny)]

Luke, don't worry about feeling stupid mate, after all the've put up with me for the past few months ;0)
(This is the second time I've posted this, I don't even know where the first one went!)

Everyone has to start somewhere. They're a helpful bunch on here and really know their stuff.

[Renato1 (Renato)]

The Skyglow filter is my favourite broadband filter for backyard viewing from a light polluted area, but I'd have thought its effect on Omega Centauri was more aesthetic than anything else in terms of actual detail (since it's meant for emission nebula).

Anyhow, if you do get one, you'll soon notice the problem that stray light hits the filter mirror and mucks up the view if your eyepiece rubber eyecups aren't good enough. Solutions include cupping your hand around the eyepiece, buying butterfly eyecups, using a beach umbrella for light blocking and putting black cloth over your head and down to the eyepiece to block out stray light. Depending on how much light your neighbours and street lights pump out into your yard, you may want to try the black cloth trick by itself (maybe use a dark cardigan/jacket).
Regards,
Renato

[Suzy]

Hi Luke,

I observe Omega Centauri regularly from my light polluted backyard and it's always been a good performer even in my 4" dob. You didn't mention what eyepiece you used that filled up the entire field of view but it sounds like you've used too high magnification. If I pop a 10mm plossl on which has a narrow 50 degree field (or is t 55?), it will fill up the eyepiece entirely and just look like a mud pie with strands running through it (much like you've described it). However, if I pop on a widefield 72 degree 10mm eyepiece, the view is incredible- the entire glob is visible with the black sky filling up the rest of eyepiece. Use lower power so you can see some dark sky around it; that will add to the contrast.

Also, don't stare at it. Roll your eye away (without moving your head) and roll it back to the glob- you should see it sparkle & some of it resolved into separate sprayed stars. If you stare at them they will just be fuzzy. It's to do with our eyes, they are not good at performing in the dark.
I am not talking about using averted vision here, just purely looking away.
However, using averted vision will make the views even better, and this is what we use for feint fuzzies to stand out. Though I don't class Omega Centauri as a faint fuzzy (I think you're using too much magnification). Yes, higher magnification can yield more contrast by darkening the sky background but it can also kill off many faint fuzzies like galaxies unless they are bright to begin with. In time, you will learn what magnification works best for these. And if the sky's transparency is poor, that high magnification will go against you too. Many people make the mistake of thinking they need to up the magnification.

How To Master The Art Of Averted Vision.

How To Hone Observing Sensitivity And Accuracy

Understanding Eyepieces

Luke, take your time and have fun. Don't worry, with some practice you will get good at this stuff!

[astro744]

Originally Posted by Suzy

Hi Luke,

I observe Omega Centauri regularly from my light polluted backyard and it's always been a good performer even in my 4" dob. You didn't mention what eyepiece you used that filled up the entire field of view but it sounds like you've used too high magnification. If I pop a 10mm plossl on which has a narrow 50 degree field (or is t 55?), it will fill up the eyepiece entirely and just look like a mud pie with strands running through it (much like you've described it). However, if I pop on a widefield 72 degree 10mm eyepiece, the view is incredible- the entire glob is visible with the black sky filling up the rest of eyepiece. Use lower power so you can see some dark sky around it; that will add to the contrast.

Also, don't stare at it. Roll your eye away (without moving your head) and roll it back to the glob- you should see it sparkle & some of it resolved into separate sprayed stars. If you stare at them they will just be fuzzy. It's to do with our eyes, they are not good at performing in the dark.
I am not talking about using averted vision here, just purely looking away.
However, using averted vision will make the views even better, and this is what we use for feint fuzzies to stand out. Though I don't class Omega Centauri as a faint fuzzy (I think you're using too much magnification). Yes, higher magnification can yield more contrast by darkening the sky background but it can also kill off many faint fuzzies like galaxies unless they are bright to begin with. In time, you will learn what magnification works best for these. And if the sky's transparency is poor, that high magnification will go against you too. Many people make the mistake of thinking they need to up the magnification.

How To Master The Art Of Averted Vision.

How To Hone Observing Sensitivity And Accuracy

Understanding Eyepieces

Luke, take your time and have fun. Don't worry, with some practice you will get good at this stuff!

The OP said he used a 9.7mm Meade Plossl. This is not a bad eyepiece and gives 123x and 0.42 deg. true field at 1200mm focal length. (Apparent field = 52 deg). I would think the Meade eyepiece is better than the 10mm Plossl supplied with most telescopes nowadays the latter eyepiece having poor contrast. The Meade would be no where as good in terms of contrast as the 10mm 72 deg eyepiece which you mention and I take to being the Pentax XW.

Note no matter what the apparent field is a 10mm eyepiece will still give the same magnification so seeing extra black sky around the object is a function of the larger apparent field, not lower magnification when comparing a 10mm Plossl to a 10mm wide field eyepiece.

Omega Centauri is a bright deep sky object and takes higher powers nicely. A magnification of 123x is not too much in an 8" telescope giving an exit pupil of 1.6mm, (apperture/mag = 200/123 or eyepiece f.l./f.ratio = 9.7/6). Anything from about 1mm to 2mm exit pupil is ideal for Omega Centauri in an 8" telescope and yes the larger apparent field eyepieces will give a more pleasing view because of the extra sky they show around the object and perhaps more so the extra contrast high quality wide field eyepieces will usually have.

If the OP has the 12.4mm Meade Plossl, try it. Other than that may I suggest an 11, 12 or 13mm Nagler as it would be ideal for Omega Centauri and many other objects. Note if wearing eyeglasses then only the 12mm eyepiece with better eye relief should be considered. Note for a bit less power and more pinpoint and tighter view of Omega, try a 16 to 17mm eyepiece of wide apparent field as the view will be very pleasing provided the eyepiece has excellent contrast.

Before buying extra eyepieces may I suggest first a visit to a dark site with the equipment at hand and see what works best and what gives the nicest contrast. Only then will you know if it is your equipment or sky conditions that is causing the poor contrast.

[el_draco (Rom)]

Originally Posted by ixian

I don't know anything about eyepieces, I've been trying to figure it out but it makes me feel stupid.

Don't sweat it. I've been doing astronomy for 40 years and still feel stupid at times. Its part of the fun of learning.

[Suzy]

Hi Astro744,

Originally Posted by astro744

The OP said he used a 9.7mm Meade Plossl. This is not a bad eyepiece and gives 123x and 0.42 deg. true field at 1200mm focal length. (Apparent field = 52 deg). I would think the Meade eyepiece is better than the 10mm Plossl supplied with most telescopes nowadays the latter eyepiece having poor contrast. The Meade would be no where as good in terms of contrast as the 10mm 72 deg eyepiece which you mention and I take to being the Pentax XW.

Thanks, I now see in his second post that he mentioned the eyepiece used. And thank you for clarification with the 52 deg fov.
I agree Omega Centauri is a bright object & can take high magnification well but from my own experience, I really do believe the small fov of a plossl at high magnification kills it as you're looking pretty much into the guts of it & is fuzzy. I didn't mention that I use XWs because I observe this object in my 4" dob with a 15mm Orion Expanse 62 deg. eyepiece and it's still a nice object despite the low power & cheaper eyepiece.

Luke, as Astro 744 mentioned, try using the 12mm on it.
Even a 15mm will give really nice views.
And think about grabbing 10mm wideview eyepiece. It doesn't have to be a premium eyepiece (but they will give you better views). It'll also help with finding things at high magnification because you have a larger field of view. Also, it'll come in handy for our other showpiece glob, 47Tuc. (NGC 104). On my 6" & 10" dobs (1200 F/L) my 10mm widefield eyepiece is the hardest working one.

Also, if you haven't already observed The Sombrero Galaxy (M 104), hurry up & take a look before it leaves our skies for this season. It's a showstopper and takes high magnification well, so use your 10mm. If the views disappoint, try on different nights as poor sky conditions can have a big impact in killing off pleasing views. I pick some very faint object than can just be made out with the naked eye, to gauge how the sky is. If I can't see it well enough, I know not to expect too much.

[The Mekon (John Briggs)]

I think Suzy is 100% correct in her advice to use a wider field eyepiece. My brother-in-law has the ubiquitous 8" dob and was viewing omega cent, from my backyard at Bowral last year. Possibly similar conditions to OP's regional city. The difference when changing from a 50 degree 10mm plossl to a 82 degree 9mm nagler was astounding. In the wider filed the whole cluster appeared much more distinct from the background sky, and of course the nagler made the stars towards the edge much clearer. Remember coma will distort the image at the edges so anything you can do to reduce this will improve the view.

[ixian (Luke)]

I actually took the scope out to a dark site; well actually more a green sky site, transitional at least. Needless to say it was fantastic, omega cent was much brighter without the sky glow.

I think my city (Toowoomba) actually has much higher light pollution than I tended to believe in the past; thirty minutes drive to the north and nebular, like Lagoon, just jumped out of the eyepiece, pretty much totally invisible in my back yard at home.

Thanks for all the advice, I'm thinking about getting in touch with SAS, who have a dark site about fifty minutes from me before I buy any more eyepieces.